The present invention relates to a high intensity, planar light source, providing flat field and uniform color temperature illumination. The source is useful for illuminating films, transparencies and like objects where high resolution is required for digital and analog imaging. The requirement arises in fields such as aerial reconnaissance, lithography, color and black and white film interpretation and enhancement. Generally any circumstance which demands bright, even illumination over a large viewing area for scanning, computer absorption or real time viewing is a candidate for the source.
Illumination devices conceal the complex and subjective technology of light, a commodity which when improperly applied can bring the most sophisticated, well understood imaging system directly to its knees. System specifications for illumination may be vague or left to the final user. Expensive digital devices are long paid for when illumination deficiencies become apparent and it is then that critical light parameters such as luminous intensity, spectral irradiance, illumination flatness, and spectral output become an unwelcome part of our vocabulary. Such parameters among others, define the limits of light source output and predict the performance of sensors and imaging equipment. Direct symptoms of an unfit light source are not limited to elongated exposure times (reduced throughput), but can also include inaccurate image characteristics, poor color rendering and, in general, otherwise unresolvable image characteristics.
Light and light measurement are among the least understood of technologies. Discussion and communication of desired operating parameters are complicated by a plethora of names for seemingly similar units of measurement. Additionally, measurements relate to "perfect surfaces" or "point sources" which are themselves located inside "perfect reflectors" or above "perfect planes". Measurements of color, for example, often appear and sometimes are subjective in nature and seemingly an art form rather than part of any science. Only those definitions which are measurable on actual functioning devices are discussed below.
Digital and Analog Sensors
The actual sensing element in a digital or analog vision system (the CCD, CID, or tube) whether used for B & W or eventually filtered for specific "colors" such as in an RGB camera, will see in "color". There are specific bandwidths or wavelengths of light which maximize the response of the sensor and provide it with an optimum operating environment. Matching the sensor to the wavelengths of light it wishes to sense is the first critical rule of effective digitizing practice and will affect the performance of the unit.
Sensor Spectral Responsivity
In order to understand the generalization of light it is first necessary to understand spectral responsivity. The complete electromagnetic frequency spectrum spans the full distance from cosmic rays down to electric power frequencies. The human eye is sensitive to only a small portion of this spectrum which is divided into three basic additive components, namely visible red, green and blue.
The eye's response is not the same to all specific colors but varies as the frequency of the light (and corresponding wavelength). It is known that camera sensors also have the characteristic known as spectral responsivity which can be represented by a responsivity curve. Responsivity curves for sensors vary in shape and in latitude, that is the accepted wavelengths, over a wide range depending upon the type of sensor, such as tube types, or solid state sensors, such as CCDs (Charge Coupled Detectors) and CIDs (Charge Induced Detectors).
A responsivity curve which defines a relatively flat response suggests that the camera will see a wider latitude of light frequencies than will the human eye. Even with a flat response, graphical analysis will reveal that altering the sensed (or illumination) wavelength from 400 to 650 nanometers will provide an increase of 50% in the spectral responsivity of the camera. This will result in increased camera response for a given light intensity (or less intensity at the new wavelength is required to achieve the same result). This suggests that if the scan time is photon dependent (scan speed is controlled by the rate at which a minimum number of photons affect the charge display wells), shifting the illuminant from 400 nm to 650 nm could reduce the scan time and thereby increase scanning throughput by 50%.
Sensor specifications often relate to a specific illumination color such as 3200K. This notation is the illumination color temperature. A definition of illumination color is given below. What is important to note here is that the illumination color specified is logically that which provides optimum performance. When filters are used in front of the sensor, the sensor responsivity curve is unaltered; however, the sensor will now see only a portion of the original illumination wavelengths.
Illumination Color
With sensor response defined, the next consideration surrounds the illumination source. In the photon sensitive camera a minimum number of photons must pass through the film and trigger each display well to assure specified performance. The more general condition of solid state and tube type sensors is not backed by "rate of scan" software. In such cases the sensor must see all of the gray shades or colors represented above the highest level of noise in the camera and imaging system itself. If there is not enough illumination available at the side of the film facing the camera, then significant losses in contrastual and spatial resolution can occur. This exercise becomes film dependent, intensity dependent, and illumination wavelength dependent. The "color" of the illumination used therefore determines the efficiency and effectiveness of the entire digital processing system regardless of whether the film is black and white or color.
The maximum density of the proposed film, the wavelength of the illumination source and the stated sensitivity of the camera sensor compensated for the lens must be taken in account. The scanning of a color film where the film itself acts as an infinitely variable filter causes the camera to modulate along the responsivity curve. Each time the various hues of blue, green, and red are individually scanned, the sensor follows its responsivity to a different efficiency index and absorbs the varying intensities of light. The component values of the light source become critical. For example, if the light source has a limited or low "blue component" the result would be exceptionally slow or underexposed by the camera due to the "blue" cycle. In the case of scanning red, the use of a highly blue slanted source with little or no red output causes the sensor to have reduced input and subjects the sensor to the danger of "seeing" heat leakage (generated deep in the source components) through the illumination port.
Measuring and Matching Illumination Color
To understand more fully the response characteristic problem in color systems it is necessary to first understand the relationships between spectral irradiance, and our previously mentioned characteristic, color temperature. There are no less than four methods of measuring the color (colorimetry) of an illumination source. They include color matching wherein a human observer actually matches physical samples, color temperature wherein actual chromaticity components are measured to provide graphical interpretation, tristimulus colorimetry where filters are used to match the tristimulus spectral response functions and finally spectroradiometric measurement over the entire light spectrum.
Because color is a psychophysical phenomenon, numerous physical observer-based experiments have been conducted over the years with the intent of achieving reproducibility in readings and reducing human subjectivity. In the 1920's a series of observer based experiments were conducted culminating in 1931 when an actual chromaticity diagram was mapped. The diagram was developed to allow a basis for color matching through the combinations of the three primary color components graphically termed x, y, and z. The 1931 Chromaticity Chart or CIE diagram (in use today) was the first time that a series of numerical coordinates could be provided to designate a specific color.
Illumination sources for most imaging systems are of course not one specific color but a bandwidth of wavelengths, a graduation of colors, each with a differing amplitude. This leads us to the concept of "color temperature", a unique application of the chromaticity diagram and a convenient method of describing the color of many common light sources.
The notion of color temperature is based upon incandescence of a perfect "black body radiator" which is heated through a temperature range (measured in degrees Kelvin) to provide reference colors. As the temperature of the block body increases, it will eventually incandesce or emit a color. A rod of steel appears as a dull red when sufficiently heated. The same is true of a tungsten light filament when sufficient current is passed. Both items have reached a point of incandescence. A special class of incandescent "black bodied" objects are assumed to have radiant efficiency of 100%, against which real world materials are measured. Tungsten, for example, has an efficiency of 40% and simulates a "black body" in color but not in its output strength for a given temperature. Each "blackbody" curve defines the spectral irradiance for a given temperature producing a family of curves (shown in FIG. 1) which identify the color sensations. The family line drawn through the maximum values is called the Plankian Locus. Note that each temperature curve has a spectrum over which it has radiation, so color temperature does not express one specific color but a bandwidth of color located about its color temperature locus.
The specification of color temperature defines completely the color distribution of a source close to the Plankian locus and can be determined by using ratios of blue and red (measured spectrometrically). The result is referenced to a calibration curve, or a direct reading color meter can be used. Such meters employ special filters with light absorbing characteristics and photocell response which duplicates the CIE diagram. Several of these designs also have microprocessors on board to calculate the color temperature directly. Illumination devices such as arc or fluorescent lamps provide "apparent" color temperatures and must be indicated as such. Such illumination devices may be lacking in certain components of the appropriate black body spectrum or may have components which are amplified. In the case of these illumination sources a three color ratio must be accomplished or one must use a three color meter equipped with "flicker" correction where applicable.
Illumination Curve Continuity
Effectively then, for black and white imagery the color temperature of the source should at least theoretically match the condition required by the spectral efficiency (or responsivity) of the camera sensor. In halide based black and white imagery, the light intensity at the sensor will change as the interscene dynamics are scanned by the camera; however, the color temperature will not. The blackened areas will merely function as an increasing or decreasing neutral density filter dependent on the darkness or lightness of the film at any given point.
In the case of color imaging the effective color and therefore the "at point measurable color temperature" will constantly be in motion at least from the camera perspective. The efficiency of the color swing and the faithfulness with which it is recorded will depend now on the faithfulness and spectral emissivity of the source itself. This faithfulness will in no small part be a function of the continuity of the spectral curve of the light source, a new variable ascertainable only by complete spectral distribution analysis (Spectroradiometric analysis).
Many image processing systems have the ability to adjust for shifts in color, (i.e. the film was exposed at 3200k but is now illuminated at 3000k). Such processing assumes the continuity and conformity of the illumination curve; however, spikes in the illumination source curve suggest that the operator may have little knowledge of the true emphasis that certain sources can place on data at unique points in the illumination spectra. Since analysis systems are being used more and more in sophisticated discrimination problems, the spectra of the illumination curve can obviously produce results which may not be represented by real data. Enhancement of generated data only enhances the problem.
Source Aging Stability
Once the proper color temperature of a source is chosen, the stability and reliability of the source must be considered. Terms such as day-light white, true white, tungsten white, and daylight fluorescent, all refer to specific if not exclusive operating conditions under which a light source may theoretically operate, but provide no information as to consistency, spectral aberrations or stability.
Imaging errors can occur when the source changes general characteristics or stability over its lifetime. An image processed today should yield the same results in hour or a week from now. Repeatability (especially long term) is sometimes beyond the capability of but a few illumination sources.
Voltage Regulation and Stability
While aging stability speaks of long term variances, short term voltage equilibrium is just as critical to the success of the operating illumination and imaging system. Voltage regulation or duty cycle fluctuations inherent in the power supply can completely alter the operation of an illumination system. All illumination systems operated on 60 hz AC will provide dramatic short term variation in output. If the frequency is high enough, (several Khz) the illumination response is not quick enough to follow the rapidly generated curve and the effect is minimized. Where possible, as in the case of planar illumination, all power should be low ripple direct current (DC). The r.m.s. ripple (the flatness of the DC waveform) should be very low (the order of 1% or less). Even a 1.5% ripple (considered reasonable for illumination power supplies) can be troublesome. This is because the normal ripple is calculated in reference to the power supply output voltage; however, the luminous (light emissive) output ripple can be higher (around 4 times higher in some cases) at the blue end of the spectrum. The high frequency spectral distribution tail is significantly more sensitive to temperature variation (and therefore instantaneous operating voltages than the red (lower) end.
Primary input voltage fluctuation (in the 120v or 240v, 50/60 cycle line) can and will affect lumen output (perhaps during a scan line) and alter color temperature. The affect of cyclical variation and input peak variation may be additive or subtractive dependent on the portion of the film being processed and whether the shift is upward or downward. Such effects on image quality are therefore unanticipated and difficult to diagnose.
Luminous Intensity
There are actually three separate systems of units used in light measurement, the CGS, the English and the SI (international convention). Many of the terms such as foot-candle and candela are very misleading. For purposes of this application, it is assumed that the film is in direct contact with the illumination port or surface. All radiation is or is nearly perpendicular to the film, and the intensity is measured directly in contact with the port surface. For continuity and common language, the units recommended to measure the light from a source should either be the footlambert or, in metric terms, candelas per meter squared. The footlambert is a measurement of one lumen of flux emanating from one square foot. One lumen illuminating from one square meter is 10.74 lux or one candlepower. There are 10.74 sq. ft. in one square meter so the conversion factor between lux and candlepower is 10.74. Thus, lux or candlepower is a measurement of illumination (illuminance) falling on a surface rather than luminance or brightness emanating from a source. Correspondingly, camera sensors are illuminated (measured in candle power or lux) by the luminant source (measured in footlamberts or candelas per sq. meter). The lumen is a measurement of radiant intensity common to both measurement systems and was subjectively developed from interpreting the illumination a perfect candle would provide through a perfect lambertian diffuser, the opposite side thereby irradiating one footlambert. Notwithstanding, it is a reproducible standard providing credence to this particular measurement unit.
There are many commercial meters on the market today which will measure in both or at least one of these units of measure. Parameters surrounding the choice of illumination will most usually be specified in some form or another in a specification sheet for the camera system (the illuminated sensor). Such a specification would be as follows: EQU Minimum Illumination: 12.5 lux at F=1.4, 3200k
Determination of Intensity Requirement
The camera, given in the example above, will respond adequately to an illumination level of 12.5 lux with an assumed quality lens operating at an f-stop of 1.4, and an illumination color temperature of 3200k. It is impractical that the camera system will operate with its lens at 1.4 exclusively but rather at several f -stops higher which would increase the illumination requirement by say a factor of eight or sixteen. The specification assumes that the system is looking directly at the light source with no film between it and the camera. It is recommended therefore to start with a densitometer and a "typical film" to determine the attenuation factor of the film, then multiply the factor by the lens-stop corrected minimum lux factor. In real practice the combined correction factors can be in the order of several thousand for certain applications. A typical application calculation is as follows:
Based on the information provided in the above specification, the minimum lux necessary to provide an image on the sensor is 12.5 lux. Given that the lens is to be operated at f 4.0 and not 1.4 the light now entering the camera is 1/8 of the original intensity (halved with each increase in f-stop). If the highest density measured on a typical film is 3.0 then the transmission will be only 0.1% of the original illumination at that point. Assuming that the camera is used in a straightforward manner (no photon dependent software or image averaging) then the minimum intensity for illumination would be: EQU 12.5 lux divided by 1/8 divided by 1/1000.
The illumination at the film surface needs to be 12.5.times.8000 or 100,000 lux! (assuming that the illumination is at 3200K)
The calculation for this low lux camera is typical. Ranges of 50,000 lux to 150 000 lux are nominal for digital imaging systems. As with most things, it is easy to stop the system down or provide neutral density filters when necessary but impossible to generate additional illumination from a system already operating at maximum.
Illumination Flatness
The above illumination calculation suggests that the minimum illumination required at any point on the film is 100,000 lux. This assumes that the flatness of illumination can provide that illumination intensity as a minimum. "Hot spots" or "shadows" will not only detract from the image quality but may also provide that some areas may be grossly underexposed. Most digital imaging systems digitize the lumination source with no film present and then subtract the image from future film sequences to compensate for illumination variance across the scene of view. This corrects for flatness consistency but will not correct for points which are below the minimum illumination requirement.
It is important to utilize the flatness coefficient specification in determining the minimum available intensity as shown in this example.
To guarantee a 100,000 lux illumination intensity at any point a 110,000 lux illumination is specified with a flatness coefficient of +/.+-.5% maximum variance. EQU 110,000 lux.times.0.95=104,500 lux
This sensor-received illumination would allow detection of the image at all points without concern regarding minimum signal levels at the sensor.
A number of light sources have been used for digital imaging techniques in the past. Fluorescent gas sources have been utilized in many applications; however, such sources must be synchronized with the scanning of the camera because the sources turn on and off with the AC excitation and the intensity of the image varies accordingly. Synchronization with the camera is a complex procedure and significantly reduces the rate of throughput. Fluorescent sources are also inherently low in intensity and do not have a constant color temperature from one end of the fluorescent tube to the other. Additionally fluorescent sources change color temperature almost immediately upon use and lose brightness within the first 10 hours of use. In summary, gas type tube sources have low brightness levels, lack uniformity in color temperature and intensity, vary intensity over the power operating cycle, and immediately suffer degradation of brightness and color temperature when placed in use.
Halogen lamps, on the other hand, are considerably more desirable. They generate a significantly higher level of brightness for their size and have a stable color temperature over the life of the lamp. The drawbacks of a halogen lamp are the considerable amount of heat which they generate in use in comparison to the fluorescent lamps, and they too vary in brightness over the power duty cycle. Another disadvantage of halogen lamps is that the light is effectively emitted from a point source, namely the filament of the lamp, and hence flatness of the field of illumination, that is, uniformity over a wide surface area, is difficult to achieve. One elaborate technique in the prior art for obtaining flat field illumination from halogen lamps employs an integrating sphere in which the lamps are mounted facing into the interior of a large sphere, and a hole or port is cut in the sphere (not facing any lamp) with a radius that is much smaller than the diameter of the sphere (usually 5-10%). The interior of the sphere is surfaced with a lambertian reflective material (diffuses light with a cosine characteristic without transmission loss), and the output illumination is sampled through the hole or port. The obvious disadvantage of such a system is size. For a planar 10 inch.times.10 inch illumination area (the minimum size for much aerial photography), the sphere approaches three feet in diameter. For larger viewing areas such as 14 inches.times.17 inches, (the size needed for reading X-rays in medical applications) the sphere may have to be 4 feet.times.6 feet in diameter. Along with the size problem is the necessity for cooling the entire structure due to the heat generated by the lamps.
Still a further problem that exists with prior sources is the difficulty of detecting the failure of only one lamp which decreases the intensity of the source but often in a little noticed manner because of the limited effect that one of a plurality of lamps will have on the total output of the source.
It is therefore, a general object of the present invention to overcome the difficulties of the prior art illumination sources and to provide a light source having many of the desirable characteristics needed for digital imaging as discussed above.
It is a further object of the present invention to provide a high intensity illumination source which possesses flatness of field, high level illumination and color temperature age and operating stability for use with digital cameras and other devices.